Common optical elements such as lenses and prisms are formed from homogeneous masses of transparent materials. Thus, the properties of the material are homogeneous throughout the entire optical element. Although several optical elements from different materials can be cemented or otherwise assembled together to form a compound element, the properties of the material within each element of the compound device are still constant. Optical theorists have realized that the performance of optical systems could be made with optical properties such as index of refraction and other properties which vary in a pre-selected fashion from place to place within the optical element. Optical elements having an index of refraction which varies from place to place within the element are referred to as "gradient index" elements.
Various attempts have been made heretofore to manufacture gradient index elements. Certain small gradient index elements can be fabricated by exposing a uniform material to a leaching process in which one or more constituents are gradually removed by diffusion from the element into a leaching solution. The reverse approach of exposing the element to a bath or vapor containing an additive so that the additive diffuses into the element can also be used. These surface-based modification techniques require lengthy treatment at elevated temperatures and under controlled conditions to produce even a small gradient in the refractive index over a small region of the optical element adjacent its surface. Accordingly, these techniques have been largely limited to fabrication of very small optical elements such as optical fibers with very small gradients in index of refraction. Further, the gradient profile is limited to a Gaussian profile, being controlled by the diffusion kinetics.
U.S. Pat. Nos. 4,907,864; 4,883,522; and 4,929,065 of Hagerty et al and U.S. Pat. No. 5,044,737 of Blankenbecler disclose fundamental advances in the manufacture of gradient index optical elements. The disclosures of these patents are hereby incorporated herein by reference. These patents disclose the manufacture of optical elements by assembling starting materials of non-uniform composition to form a starting assemblage having different compositions at different locations in the assemblage. For example, glass plates, each of a different composition, may be stacked one atop the other to form the starting assemblage. In an analogous manner, the starting assemblage may include a mass incorporating glass frits or powders of various compositions arranged to that the different compositions are located at different points. The starting assemblage is then subjected to a diffusion process, typically at an elevated temperature, so as to fuse the various starting materials into a solid mass, commonly referred to as a "blank", which can either be used directly as an optical element, or, more preferably, ground to a desired shape or otherwise treated to fashion it into a finished optical element. During the diffusion process, certain constituents tend to diffuse from regions of the assemblage where the concentration is high to regions where the concentration is lower. Thus, after the diffusion process has operated for a finite time, the resulting blank has a smooth gradation in composition and hence has optical properties which vary smoothly from point to point within the blank. Because the processes taught by these patents do not depend upon diffusion to or from the surface of the blank, but rather involve formation of a large gradient within the bulk of the blank on the order of 0.08 and larger (also termed as a "macro-gradient"), they can be used to form blanks, and hence optical elements, of essentially unlimited size and with substantial gradients in optical properties. Further, the profile of the gradient may be pre-selected by judicious choice of compositions and thicknesses of plates or frits, resulting in, for example, linear, quadratic, cubic, and other profiles. Such profiles permit fabrication of optical elements in which spherical aberrations are internally corrected, without the need of external correcting elements, for example. Thus, the processes taught in these patents accordingly represent major advances in the art of fabricating gradient index optical elements, and provide gradient index optical elements which were not previously available.
Despite these major advances in the art, there remains a need for still further improvements. Prior lead-silicate glass series having, for example, an index of refraction ranging from about 1.563 to 1.807, evidence a mismatch in thermal expansion coefficient after diffusion, causing fracture during fabrication and wavefront distortions in the gradient glass element. Even though all these glasses initially have well-matched thermal expansion coefficients, the composition change due to the multi-component diffusion process apparently results in thermal expansion coefficient change which consequently causes a mismatch in the thermal expansion coefficient of the gradient glass element.
Thus, a series of compositions based on lead silicate is required which permit fabrication of a lens blank having a substantial change in the index gradient, yet with each glass composition possessing a thermal expansion coefficient that is substantially the same for each composition, even after diffusion. In this way, plates or frit of the compositions, when layered atop one another, may be subjected to the afore-mentioned diffusion process, annealed, and cooled without evidencing fracture or wavefront distortions. Such glasses would have essentially no strain or birefringence within the element.